The present invention relates to a process for the recovery of sodium carbonate from solutions containing sodium carbonate and sodium bicarbonate and, particularly, from solutions obtained from the solution mining of trona ore deposits.
Sodium carbonate, also called soda ash, is an important, high volume chemical produced in the United States and used in the manufacture of glass, chemicals, soaps and detergents and aluminum, as well as in textile processing, petroleum refining and water treatment, among many other uses.
In the United States, almost all sodium carbonate is obtained from subterranean deposits of naturally occurring trona ore. The largest known trona ore deposits in the United States are located in Green River, Wyo. and are typically about 800 to 3000 feet below ground level; these trona ore deposits are actively mined by several companies. Trona ore primarily comprises sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O), and a typical analysis of crude trona ore being mined at Green River, Wyo. is as follows:
Constituents Weight Percent (wt %) sodium sesquicarbonate 90 sodium chloride (NaCl) 0.1 sodium sulfate (Na.sub.2 SO.sub.4) 0.02 organic matter 0.3 insolubles (clay and shales) 9.6
Trona ore is recovered from these subterranean deposits for further processing into soda ash by conventional mechanical mining techniques, also called dry mining, such as room and pillar and long wall methods or by any of several various solution mining methods. The Green River, Wyo. trona ore deposits are presently being commercially mined by both mechanical and solution mining processes. Mechanical mining methods are relatively costly and leave unrecovered a significant fraction of the trona ore in the beds being mined, so solution mining processes present an economical alternative to mechanical mining.
Solution mining allows the recovery of sodium carbonate values from subterranean trona ore deposits without the need for sinking costly mining shafts and mechanically extracting the ore using a mining crew in the mines. Solution mining can be accomplished by injecting water or other aqueous-based solution via a drilled well hole into a deposit of trona ore, allowing the solution to dissolve as much of the soluble ore as possible, pumping the solution via a drilled well hole to the surface, and processing the solution to recover the dissolved ore values from the solution in the form of sodium carbonate or other related sodium based chemicals. Solution mining methods are also useful for recovery of trona ore from depleted ore deposits that have previously been mechanically mined.
Numerous solution mining techniques have been described in the prior art: see, for example, U.S. Pat. Nos. 2,388,009 of Pike; 2,625,384 of Pike; 3,050,290 of Caldwell et al.; 3,119,655 of Frint et al.; 3,184,287 of Gancy; 3,953,073 of Kube; 4,401,635 of Frint; and 5,043,149 of Frint et al. In these prior art solution mining processes, a primary objective was to maximize solubilization of the trona ore in the mining solvent or to otherwise provide a concentrated solution, or brine, for processing to recover soda ash. These prior art approaches included use of heated aqueous solvents or of sodium hydroxide-containing solvents or fortification of a recovered brine with added alkali values. The resulting highly concentrated solutions could then be more economically processed into soda ash, for example, by using conventional soda ash recovery techniques such as the sesquicarbonate process or the monohydrate process. The sesquicarbonate process and the monohydrate process were originally developed to process mechanically mined trona ore into soda ash, so they do not always necessarily provide the methods best suited for recovering soda ash from solution-mined brines.
The sesquicarbonate process involves dissolution of mechanically mined trona ore in a recycled hot mother liquor containing excess normal carbonate over bicarbonate in order to congruently dissolve the sodium sesquicarbonate in the trona; clarifying and filtering the solution to remove insoluble matter present in the ore; passing the filtrate to a series of vacuum crystallizers to crystallize pure sodium sesquicarbonate as the stable crystal phase, by evaporation of water and cooling; recovering crystallized sodium sesquicarbonate from the crystallizer slurry; recycling the crystallizer mother liquor to dissolve additional crude trona, and calcining the recovered sodium sesquicarbonate crystals at elevated temperature to convert the sesquicarbonate to soda ash.
The monohydrate process was developed in response to the need for a more dense soda ash than that produced by the sesquicarbonate process. In the monohydrate process, mechanically mined trona ore is calcined at elevated temperature to convert it to crude sodium carbonate; the calcined crude sodium carbonate is dissolved in water, the resulting sodium carbonate solution is clarified and filtered to remove insolubles; the clarified filtered solution is then processed in an evaporative crystallizer circuit to remove water and crystallize sodium carbonate monohydrate. The crystallized sodium carbonate monohydrate is recovered and calcined to produce a dense soda ash.
Methods other than the conventional sesquicarbonate process and conventional monohydrate process have been described in the prior art for processing of solution mining liquors or of other similar brines containing alkali values; see, for example, U.S. Pat. Nos. 3,264,057 of Miller, 3,273,959 of Miller; 3,273,958 of Peverley; 5,283,054 of Copenhafer et al.; and 5,609,838 of Neuman et al. These prior art methods are typically complex procedures, involving multiple steps in which various forms of sodium carbonate are crystallized, and these multiple crystallization operations add significantly to the overall economic cost of these soda ash recovery processes.
The present invention provides a process for the production of soda ash that is more direct and economical than the complex crystallization procedures described in the prior art and that may be used with a wide variety of aqueous mining liquors, without the need to fortify such aqueous mining liquors with additional alkali values prior to recovery of the soda ash.